In clocks or clock-based integrated electronic devices, multiple representatives of the same clock or data signal are often derived from the same source signal. Although the buffers and interconnections used for this purpose are designed with great care, there is often a variation between the same rising or falling edges of the multiple representations of the same clock or data signal. The time shift or offset between the signals is known as skew. The amount of skew is typically measured by an external automatic test equipment (ATE).
To measure skew on automatic test equipment, the ATE has to provide a precision time measurement system or a repeatable signal edge placement. Generally, all time measurement units that can measure time delays can also measure output skew. The accuracy of the measurement depends on the repeatability and precision of the time measurement unit. It is also possible to measure output skew by analyzing edge placement results. The accuracy in this case depends on the edge placement resolution and on the edge placement repeatability. However, the time measurement system on the ATE is sometimes not accurate enough to measure output skew in the low ps range. Furthermore, often there is no time measurement system available at all on the ATE. If there is a time measurement system, it must often be shared between sites during multi-site testing, which makes multi-site testing extremely difficult. On some ATE test systems it is also not possible to control edge placement timing precisely and the accuracy of edge placement is not accurate enough for output skew measurement. When systems do have an accurate time measurement system or an accurate edge placement, they are usually very expensive. Further, it is difficult to design a test load board for output skew measurement. This is because the specified load for the device is often not the same as the impedance of the measurement system, which can have a negative influence on the measurement results. Also, the lines of the signal channels where the output skew should be measured must have exactly the same length and variations in board manufacturing negatively influence the output skew results.